Qualitative identification of hydrologically different water sources used by plants in rock-dominated environments

Sources of water used by plants in rock-dominated environments are far less clear than those in relatively homogeneous soils, largely because plants in rocky environments normally expose to more complex substrate conditions and their potential water sources are hard to locate and sample. In the current study, potential water sources for plants growing on cliff-face (a typical kind of rock-dominated environment) were roughly separated into two water pools based on their hydrological differences. A variable pool that is composed of water derived from recent precipitation that has rapid fluctuations in water content due to recharge by precipitation and depletion by transpiration, and a constant pool that is characterized by slower changes in water status because it can only be recharged by precipitation under certain conditions and depleted by transpiration of deep-rooted species. This study was conducted in a typical karst catchment of Northwest Guangxi, Southwest China. Stem water samples for the most common dominant tree species of cliff-faces and rainwater for each rain event were consecutively collected throughout the growing season. A co-occurring shrub species, which was identified to have a shallow root system, was further selected as a reference species. The reliance on different water pools was qualitatively identified based on the deviation of stem water isotope values between current and last sampling times, as well as its relationship to the isotope values of rainfall between these two sampling times. Our results showed that the shrub species continuously relied on water from the variable water pool, which was consistent with its shallow root system. While the dominant tree species shifted from the constant pool during the early wet season to the variable pool during the mid-wet season, then back to the constant pool during the early dry season. Moreover, the divergent reliance on different water pools by the two species was further supported by the species-specific dynamics in the efficiency in water use by various plants (reflected by leaf carbon isotope ratio). Our results provide new insights into the linkages between plant transpiration and hydrologically different water pools, which would be helpful in understanding the hydrological importance of different plant species.